A Leviathan-class vessel in dock as of 3057, presumably the CGB Leviathan

The original Leviathan-class Dreadnought was designed by Clan Ghost Bear, with the help of Clan Snow Raven. No ship was ever built to match the original specifications, but several sub-classes of the Leviathan have emerged:

Leviathan Transport — Clan Snow Raven started construction of two Leviathans (the Leviathan and the Great Bear) for the Ghost Bears in 3042. Halfway during construction the Ghost Bears decided to alter the design specs and have the hulks completed as armed transporters instead. Thus, the design specifications were altered and the vessels were built as the Leviathan (Heavy Transport) subclass. They saw use in Clan Ghost Bear's migration to the Inner Sphere.

Leviathan II — After the Clan had completed the move, the ships were reconfigured into battleships, but upgrading the heavy transports to designted battleships resulted in a battleship design that was different from the original Leviathan design—the Leviathan (Battleship) subclass, also referred to as Leviathan II.Following the conversion of the Leviathan and Great Bear, a third vessel (Rasalhague) was constructed straight to Leviathan II specifications by the Ghost Bears.

The Leviathan was first depicted on the cover of Technical Readout: 3057 (a Technical Readout heavily focused on space ships). However, the Leviathan class was not actually included in the book, leaving many fans to wonder about the mystery vessel shown on the cover. The in-universe date of the book suggests the depicted ship must be either the Leviathan or the Great Bear, as construction of the third Leviathan-class vessel was not begun until around a decade later. Later official miniatures art shows the Great Bear sporting a red, not blue, background for the Clan crest painted on the hull, suggesting the ship depicted is the Leviathan.

No game stats have ever been published for the original Leviathan. The known data from the transport and battleship subtypes does establish its mass (as a different mass would require a different drive core) and strongly suggests that it would have had eight docking hardpoints; beyond that, no hard data is available. Even the length of the transport version (1,623 m) and the battleship upgrade (1,700 m) is different, providing only a ballpark estimate.